The present invention relates to a thermally developable light-sensitive material, and particularly to a thermally developable light-sensitive material which provides pure black tone images and whose shelf life (i.e., the ability of the thermally developable light-sensitive material to retain photographic properties initially possessed even after prolonged storage) and developing speed are improved.
A variety of methods which comprise subjecting photographic materials containing light-sensitive components such as silver halide or the like to a so-called dry processing by heating to thereby obtain an image are known. Of these light-sensitive materials which can form photographic images using dry processing, the most common one is a thermally developable light-sensitive material as described in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,707,377 and 3,909,271, in which an oxidation-reduction image forming composition comprising, as essential components, organic silver salt oxidizing agents (for example, silver behenate), photocatalysts such as light-sensitive silver halide, and reducing agents (for example, 2,2'-methylenebis[4-methyl-6-t-butyl]phenol) is utilized. While the thermally developable light-sensitive material is stable at ambient temperature, after exposure to light, the organic silver salt oxidizing agent and reducing agent present in the light-sensitive layer undergo, when heated generally at temperatures of higher than about 80.degree. C., preferably greater than about 100.degree. C., an oxidation-reduction reaction due to the catalytic action of the photocatalyst which is present in proximity to the organic silver salt oxidizing agent and reducing agent to thereby form silver. The exposed areas of the light-sensitive layer are rapidly darkened so that a contrast is formed between the unexposed areas (background) to form an image.
In most cases, the image provided by the thermally developable light-sensitive material using the above described silver salt oxidizing agent is yellow brown in color. However, it is known that, by incorporating a color toning agent therein, the thermally developable light-sensitive material can be improved so as to obtain an image having a good black tone. U.S. Pat. No. 3,457,075 discloses that phthalazinone can be employed alone as a color toning agent. However, when used alone, phthalazinone exhibits poor shelf life, particularly at high humidity.
The use of 2-(hydroxymethyl)-1(2H)-phthalazinone as a toning agent precursor is disclosed in U.S. Pat. No. 4,125,403. 2-(Hydroxymethyl)-1(2H)-phthalazinone has been found to yield phthalazinone according to the following thermal decomposition reaction: ##STR1## 2-(Hydroxymethyl)-1(2H)-phthalazinone was found to be desirable for several reasons, chief among them being (1) minimization of fog generated during solution aging and shelf storage of coated photothermographic products with little sensitivity loss, (2) improved solubility over phthalazinone, allowing better retention in coated products, and (3) absence of sublimation, which fouls most commercially available reader-printer apparatus. However, the appearance of the phthalazinone toner by means of the thermal decomposition reaction was found to be too slow for most commercially feasible photothermographic formulations.